Obstructive pulmonary disease, including asthma, emphysema, or chronic bronchitis, afflicts more than 25 million individuals in the United States and accounted for over 17 million physician office visits in the mid 1990's. Current estimates for the total cost of these diseases are in excess of $20 billion. These diseases are increasing in prevalence due to myriad causal factors, but principally driven by smoking.
While a chronic disease, the hallmark of asthma is acute episodes of difficulty breathing created by an acute constriction of smooth muscles lining the bronchi (the passage ways for air in the lungs), reducing the diameter of the airway and increasing the resistance to air flow. Bronchial constriction in asthma is “reversible” in that the acute constriction can be reversed by bronchodilation medication or by the passage of time (after removal of the irritant that elicited the constriction). However, asthma chronically exhibits itself as inflammation, hypertrophy, or hyper-excitability of the smooth muscles.
Emphysema and chronic bronchitis are different diseases than asthma, but can be related by the same causal factor and concomitant appearance in the same or similar individuals. Both emphysema and chronic bronchitis are predominantly caused by smoking and usually both exist in the same individual, hence they can be lumped together under the umbrella term Chronic Obstructive Pulmonary Disease (COPD). However, the diseases are very different and manifest themselves quite differently. While most subjects exhibit some amount of both diseases, a subject can be categorized by which condition is predominant in the subject's anatomy.
In emphysema, long term exposure to smoke or other noxious substances can result in a primary breakdown of the lung parenchyma (alveoli, etc.). Normal fine alveoli can break down and form large open “holes” (bullea), which in turn can result in reduced surface area for gas exchange, sapping of inhaled air flow from healthy lung tissue, or reduced anchoring of bronchi that can result in airway collapse.
In chronic bronchitis, irritation of the bronchi can result in inflammation, hypertrophy, or constriction of the smooth muscles lining the bronchi, or excessive mucus production that can clog the bronchi. While the smooth muscle contraction in chronic bronchitis is not as “reversible” as that exhibited in asthma, there is usually a significant degree of reversibility and bronchodilator medications can be used as a first line of therapy.